(1) Field of the Invention
The present invention relates generally to sealing devices, and more particularly to a retractable sealing device activated by a shape memory alloy ring.
(2) Description of the Prior Art
O-ring seals are used extensively to provide a pneumatic/hydrostatic seal between adjacent parts. In a typical application, a Tomahawk Capsule Launching System (CLS) is inserted in the missile tube of a submarine with dual o-ring seals providing the seal between the CLS and the missile tube. While this arrangement provides an effective seal in a static condition, the o-rings are susceptible to damage during installation and removal of the CLS from the missile tube. In order to provide an effective seal, the o-rings, in their free state, protrude beyond the outer diameter of the CLS. As a result, the o-rings suffer from abrasion damage as the CLS is moved into and out of the tube. While minor abrasion is common in o-ring seals between moving parts, the abrasion damage to the CLS o-rings typically results in a loss of the seal between the CLS and the missile tube due to the size of the CLS and o-rings.
In such applications where abrasion damage can lead to loss of an effective seal, a lip seal may be provided. A lip seal typically consists of an o-ring partially surrounded by an elastomeric material less susceptible to abrasion damage. When the CLS with a lip seal is inserted into the missile tube, the material surrounding the o-ring deforms to allow the o-ring to compress, thus forming a tight seal between the CLS and the missile tube. Though superior to the o-ring alone in resisting abrasion damage when the CLS is inserted into the missile tube, lip seals do suffer from damage during removal of the CLS from the missile tube. A seal which would be retractable when the CLS is inserted into or removed from the missile tube would prevent such damage from occurring.
A number of prior art devices have used shape memory alloys to provide retractable seals. U.S. Pat. No. 4,515,213 to Rogen et al. provides a packing tool for sealing spaces between the wall of a wellbore and tubing inserted into the wellbore. A sealing element is supported about the tubing. The sealing element contains a helical spring of shape memory alloy which maintains a radially contracted condition below a predetermined temperature. The tubing and sealing element are inserted into the wellbore. The shape memory alloy is heated and expands radially outward away from the tube and against an elastomeric material which maintains the seal between the wellbore and the tubing. A gripping element is also provided which consists of a shape memory alloy helical spring wound about the tube which expands in a longitudinal direction to force a wedged shape gripper against the wellbore. While the helical spring shape memory alloy retractable seal of Rogen et al. could be adapted for use on the CLS, the CLS would require substantial modifications to accommodate this seal. The size of the helical spring shape memory alloy element in the Rogen et al. seal precludes its use in replacing most standard o-ring seals without modifying the slots containing the o-rings.
U.S. Pat. No. 4,773,680 to Krumme provides a pipe coupler which utilizes retainer rings of shape memory alloy to both affix the pipe ends to the coupler and to seal the coupling. The retainer rings are deformed to form a concave-convex surface or a section of a conic. When heated, the rings return to their flat shape, biting into the metal of the coupling and the pipe. This action forms a seal and prevents the pipe from being removed from the coupling. While the Krumme coupler may be adapted to seal the CLS within a missile tube, it would damage both the CLS and the missile tube in forming the seal.
U.S. Pat. No. 5,132,873 to Nelson et al. provides a diaphragm sealing apparatus for sealing of an electronic component connected to a mating fluid heat exchanger. The diaphragm has an opening shaped to fit about the heat exchanger and forming a sealing lip. A clamping ring, which expands and contracts as a function of temperature is placed around the lip of the diaphragm and is subjected to a temperature to shrink the clamping ring against the lip and heat exchanger. The clamping ring arrangement of the Nelson et al. apparatus is used to hold the heat exchanger within the diaphragm by deforming against the heat exchanger. When a pressure seal is required, the clamping ring deforms into a compressible, soft metal ring provided on the heat exchanger. In either case, the clamping ring is attached to and deforms the diaphragm to connect the heat exchanger to the diaphragm. As with the Krumme coupler, the deformation of either the CLS or missile tube would be unacceptable.